The present invention relates to a throttle valve control device for an internal combustion engine, and more particularly to a throttle valve control device with a motor-driven throttle valve.
A conventional throttle valve control device is disclosed in JP-A-63-150449 (1988), and U.S. Pat. Nos. 4,947,815 and 4,735,179 (the last one being owned by the same assignee as this application).
As discussed in the above documents, one of the most important things in a throttle valve device driven by an actuator, such as a DC motor or a step motor, is that the minimum safety can be secured even against a fault in the actuator.
With the aforesaid "minimum safety", the following is meant; i.e., even in the event of occurrence of a fault in a throttle valve actuator, an engine can produce torque of a certain magnitude, whereby an automobile can travel at a predetermined speed, e.g. 30 km/h or so, to reach a nearby repair shop. According to circumstances, however, the higher speed, e.g., 80 km/h, may be required for such an emergency speed. That is, it is desired that, even in the event of occurrence of a fault in a throttle valve actuator, a throttle valve is always kept at a certain opening so that an engine can produce torque large enough to allow an automobile to travel at a desired emergency speed.
By way of example, assume a case where electric power for a throttle valve actuator is abruptly cut off. In such a case, a throttle valve is rapidly returned by a return spring to a full close position or an idle position, with the result that a traveling automobile is decelerated drastically.
Further, with respect to .about.a full close position" and "an idle position", more detailed explanation will be given below in relation to the description of the embodiment of the present invention. For the present, however, only "a full close position (or opening)" is used.
Then, after the throttle valve is returned to the full close position, the automobile can no longer continue to travel at a desirable speed, because the engine can not produce sufficient torque with such opening of the throttle valve.
Further, in an motor-driven throttle valve device, an initial position of a throttle valve when an engine does not operate, i.e., a full close position, is necessary to be selected at such a value that re-start torque of a driving motor never becomes large. The reason therefor is as follows.
If the throttle valve is allowed to stand at the full close position for a long period of time after the engine stops, combustion products, such as tar and carbon, accumulate and adhere between the throttle valve and an inner wall of an intake pipe so that the throttle valve is fixed to the inner wall and experiences difficulty in being opened again by the torque of the driving motor.
Therefore, the conventional throttle valve device was provided with a stopper called a default stopper. This stopper serves to prevent a throttle valve from being at the full close position, when a fault occurs in the driving motor and the throttle valve is rotated by a return spring in the closing direction.
Therefore, the default stopper is different in its function from a stopper called a full close stopper (or an idle stopper), which is provided at a certain position to prevent a throttle valve from sticking into an inner wall of an intake pipe to secure a rotational speed for idling.
Even if a driver releases his foot from an acceleration pedal to temporarily stop an automobile because of a traffic signal, the throttle valve must be slightly opened to keep a rotational speed for idling so that the engine is prevented from stalling. With such a rotational speed of the engine, however, an automobile can never travel at the speed of 30 km/h, for example.
As already mentioned, in order to attain the aforesaid minimum safety, an automobile is required to be able to still travel at a desired emergency speed, even if a throttle valve driving motor has a fault. Usually, a full close stopper (or idle stopper) is provided at the position of 1.degree..about.2.degree. prior to the completely closed position of a throttle valve. Here, it is meant with the "completely closed position" of a throttle valve that the throttle valve completely chokes an intake pipe so as to block all air flow therethrough.
On the other hand, the default stopper is provided at the position that is further shifted by 3.degree..about.11.degree. in the opening direction from the position of the full close stopper. With the opening of the throttle valve defined by the default stopper as above, an automobile can travel at the speed of 30 to 80 km/h (the travel under such a condition as described above is often called a limp-home travel (or operation)).
Under the condition of the limp-home travel, the opening of the throttle valve is maintained at a value to attain the aforesaid minimum safety, but the opening can be no longer adjusted, even if an acceleration pedal is operated. Therefore, the control of the engine output is done by switching over a control mode of the ignition timing to one prepared for the limp-home operation, and/or by switching over a control mode of the fuel supply to one prepared for the same.
If an automobile temporarily stops because of the traffic signal, for example, the rotational speed of the engine will become very high. Namely, the engine gets into a runaway condition. This is because under the limp-home operation, the throttle valve is kept at the opening which is much larger than that of the full close position. Therefore, an adequate instruction for the ignition timing and/or the fuel supply must be provided so as not to cause such a runaway condition in the engine, taking account of the fact that the automobile stands still, a rotational speed of the engine, information of a fault in a throttle valve motor, and so on.
Of course, if a throttle valve motor operates normally, the default stopper is unnecessary and awkward, because desirably the motor can control the opening of a throttle valve without any limitation between the position defined by the full close stopper and the full open position, in proportion to the depression amount of an acceleration pedal.
In the prior art, this point was dealt with in the following manner. Namely, the position of a default stopper is biased by putting it on some kind of elastic body like a spring or rubber. This elastic body has the elastic force being capable of balancing with the force of a return spring, when the throttle valve is at the position defined by the default stopper. Therefore, the throttle valve is never closed further by force of the return spring only.
Then, in the conventional approach, the throttle valve device is so designed that if a throttle valve motor operates normally, a throttle valve is controlled by force to the full close position against the elastic force as described above. That is to say, the elastic body, on which the default stopper is provided, is displaced by the torque of the motor in the direction of the full close position, and the throttle valve can be closed at the full close position.
During an engine idling operation, the throttle valve motor is necessary to produce the torque overcoming the force of the elastic body, whereby the throttle valve can be rotated from the opening of 4.degree..about.13.degree. to the opening of 1.degree..about.2.degree.. For the throttle valve motor, therefore, force caused by the elastic body is preferably as small as possible. If the elastic force is small, the throttle valve motor can produce only the smallest torque necessary which reduces the load on a battery.
This is desirable especially during the idling operation of the engine, because a charging generator does not produce sufficient output power in that duration. Namely, the throttle valve motor must continue to produce torque in order to keep the throttle valve at the full close position or the idle position against the default spring. Therefore, it is desirable that the spring force of the default spring is as small as possible.